GB2058434A

GB2058434A - Optically readable record and read apparatus therefor

Info

Publication number: GB2058434A
Application number: GB8028056A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1979-09-03
Filing date: 1980-08-29
Publication date: 1981-04-08
Also published as: DE3032769C2; DK371780A; DD153012A5; ES8105108A1; FR2464532B1; BR8005553A; IT1132631B; DE3032769A1; ZA805225B; IT8024434A0; NL7906576A; SE8006065L; US4325135A; SE445960B; JPH0121543B2; AU6192380A; GB2058434B; ES494683A0; CA1150409A; AU537557B2

Description

1 GB 2 058 434 A 1 The invention relates to a record carrier containing

information in an optically readable information structure, which comprises trackwise arranged information areas which in the track direction alternate with intermediate areas, laterally adjacent track portions differing from each other in that they comprise information areas of a first type and information areas of a second type respectively. The invention also relates to apparatus for reading said record carrier.

Such a record carrier and apparatus are known, inter alia from Netherlands Patent Application No. 78 06378 which has been laid open to public inspection. The known record carrier may contain a television program, whilst the information may be encoded in the frequency and/or the dimensions of the information areas in the track direction. These information areas are constituted by pits pressed in the carrier surface. The dimensions, except those in the track direction, of the information pits may be the same for the entire information structure. It is alternatively possible that the information is encoded in digital form, in which case the information areas also have the same dimensions in the track direction.

A specific combination of the information areas and intermediate areas then represents a specific combination of digital zeros and ones.

For optical record carriers a maximum information density is pursued, ie. a maximum playing time for a carrier with a television program. For this purpose, adjacent tracks should be arranged as closely as possible to each other. However, the lateral distance between the tracks cannot be made arbitrarily small. For known record carriers, in which the information areas of the adjacent tracks have the same geometry, except for the dimension in the track direction, all these information areas influence the radiation of the read beam in the same manner. The read spot formed on the information structure by the read beam is a diffraction- limited radiation spot with a specific intensity distribution. The half- value diameter of this spot, i.e. the distance between two points in the spot where the intensity equals 1 /e 2 of the intensity in the centre of the spot, is of the order of the track width. This means that, even in the case of correct tracking of the read spot, an amount of radiation will be incident beyond the track to be read, and may even be incident on adjacent tracks. The amount of radiation on the adjacent tracks increases as the track distance decreases. A specific portion of the radiation which is incident on the adjacent tracks and which is modulated by the information areas of these tracks can reach a radiation detector which is adapted to receive the radiation which has been modulated by the track to be read. This effect, the crosstalk effect, determines the minimum distance between the tracks.

SPECIFICATION 65 Optically Readable Record and Read Apparatus Therefore

In Netherlands Patent Application No. 78 06378, which has been laid open to public inspection, it is proposed to increase the information density by giving the information pits of the adjacent tracks, i.e. the information areas of a first and a second type respectively, different depths and by reading these tracks with beams of different wavelengths. The depths and the wavelengths have been selected so that the information pits of a first track produce a maximum modulation in a beam of a first wavelength, whilst the information pits of adjacent, second, tracks hardly influence said beam, i.e. are hardly observed by said beam. The last-mentioned pits do produce a maximum modulation in a beam of a second wavelength, which last-mentioned beam in its turn is hardly influenced by the information pits of the first track. The tracks can then be arranged substantially more closely to each other, without excessive crosstalk.

However, this proposal presents some practical problems. First of all two radiation sources are required for producing two beams of different wavelengths, so that the read apparatus becomes more intricate. Secondly, for a satisfactorily separated read-out of the two types of pits, comparatively deep pits, of the order of a few times the wavelength of their associated read beam, will have to be formed with an accuracy of the order of a tenth of the wavelength of the read beam. This is a technologically difficult problem.

It is an object of the present invention to increase the information density in a record carrier for information, such as a television program, an audio program, or digital information, for example from and for a computer, while mitigating said disadvantages. To this end, the invention provides a record carrier containing information in an optically readable information structure, which comprises trackwise arranged information areas which in the track direction alternate with intermediate areas, laterally adjacent track portions differing from each other in that they comprise information areas of a first type and information areas of a second type respectively, wherein all the information areas are elongate, the information areas of the first type have such a geometry that in a first read beam component, whose direction of polarization is parallel to the longitudinal direction of these information areas and whose effective wavelength is at least of the order of magnitude of the width of the information areas, they produce a maximum modulation and, moreover, in a second read beam component, whose direction of polarization is transverse to the longitudinal direction of the information areas and whose effective wavelength is equal to that of the first read-beam component, produce a minimum modulation, and wherein the information areas of the second type have such a geometry that they produce a minimum modulation in the first read beam component and moreover produce a 2 GB 2 058 434 A 2 maximum modulation in the second read-beam component.

All information areas being elongate is to be understood to mean that over the entire record carrier the dimension in one direction (the longitudinal direction) of these areas is at least of the order of one-and-a-half times as great as the dimension transverse to this direction. Preferably, the lengths of the information areas are at least twice as great as the effective wavelength used. The polarization effects employed in accordance with the invention may begin to occur if the lengths of the information areas are approximately one and a half times their width. In previous disk-shaped record carriers proposed by the Applicant, containing the same amount of information per track circumference, the average length of the information areas was proportional to the track radius. For tracks an the inner side of the record carrier the average length of the information areas was comparatively short and approximately equal to the width of the areas.

The information structure of the record carrier may be a phase structure. The information areas may then be constituted by pits pressed in the record carrier surface or by hills projecting from this surface. The information structure may alternatively be an amplitude structure. The information areas are then for example non- reflecting information areas in a reflecting surface. 95 Furthermore, the information structure may be a structure which is intended to be read in reflection or a structure which is intended to be read in transmission.

The direction of polarization of the optical read 100 beam, which is a beam of electromagnetic radiation, is to be understood to mean the direction of the electrical vector, the E-vector.

The effective wavelength of the read beam is the wavelength at the location of the information 105 structure. If the information structure is covered with a protective layer having a refractive index n, the effective wavelength is the wavelength in vacuum divided by n.

In general, when reading the information structure under consideration, which may be regarded as a diffraction structure, steps can be taken to ensure that if the centre of the read spot coincides with the centre of the information area destructive interference occurs between the zeroorder beam and the first- order beams. The output signal of a radiation-sensitive detector, which is adapted to convert the read beam into an electrical signal, will then be a minimum if the centres of the read spot and of an information area coincide, and a maximum if the read beam is projected in between two information areas. For a satisfactory modulation of the detector signal the information areas should have a specific phase depth. The phase depth of the information structure is to be understood to mean the difference between the phase of the zero spectralorder and one of the first spectral orders, formed by the information structure, if the centre of the read spot coincides with the centre of an information area. In a first-order approximation it may then be assumed that the different first orders have the same phase. The phase depth depends on the geometry of the information areas, in the case of information pits specifically on the geometrical depth of said pits and on the angle of inclination of the walls of the pits.

Which phase depth is optimum when reading a specific information structure depends on the read method being used. An optical information structure may be read in accordance with the central-aperture read method or in accordance with the so-called differential read method. In the case of the first- mentioned read method all radiation coming from the record carrier and passing through the pupil of the read objective is concentrated on a single detector. In the case of the differential read method two detectors are employed which detectors are arranged in the so- called far field of the information structure and which are arranged after each other viewed in the track direction. The difference signal obtained by means of these detectors represents the information being read. The far field of the information structure may be represented by a plane-in which the centres of gravity of the subbeams formed by the information structure, in particular the zero-order subbeam and the firstorder subbeams, are separated. The optimum phase depth OCA for an information structure which is adapted to be read with the central aperture method is approximately 1800, whilst the optimum phase depth 0,., for an information structure which is adapted to be read with the differential method is approximately 1101.

In accordance with the invention use is made of the fact that when elongate information areas are read with a read beam whose effecdve wavelength is of the order of magnitude of the width of the areas, the direction of polarization of the readheam will play a part. It has been found that for the information structure under consideration, information pits, if read with a parallel-polarized read beam, i.e. with a beam whose E-vector is parallel to the longitudinal direction of the pits, effectively seem to be less deep, or in other words have a smaller phase depth, than the same pits if read with a perpendicularly polarized read beam. In order to.

obtain a desired phase depth for an optimum. read-out, in the case of the information structure under consideration, the information pits, should be effectively deeper in the case of reading with a pdrallel-polarized read beam than in the case of reading with a perpendicularly polarized read beam. Information pits which have been optimized for read out by a parallel-polarized read beam are generally not optimized for read-out with a perpendicularly polarized read beam and may even be geometrically dimensioned so that they are hardly observed by the last-mentioned beam. Obviously the same applies to information hills. When the information areas of two adjacent tracks are dimensioned for two mutually perpendicular directions of polarization, the track 1 3 GB 2 058 434 A 3 distance may be considerably smaller, for example twice as small, relative to the distance between two tracks of known record carriers which comprise only one type of information areas, without an increased risk of crosstalk. The information density may then be increased for example by a factor two.

The polarization effects are greatly determined by the optical contrast between the information areas and their surrounding and by the sharpness of the edges of the information areas. The optical contrast is determined by the extinction coefficient and the refractive index of the material of the information layer. This layer is for example a metal layer. The polarization effects are smaller in the case of reading in transmission than in the case of reading in reflection, though still sufficient to be used for differential read-out in transmission.

In a first embodiment of a record carrier in accordance with the invention, in which the longitudinal directions of the two types of information areas coincide with the longitudinal direction of the tracks in which said areas are situated, the two types of information areas differ 90 from each other in that at least one of the geometrical dimensions of said areas, which are not determined by the information stored, differ.

The information areas can be made to differ from each other by making the maximum width, i.e. the width in the plane of the intermediate areas, different. However, in practice, in the case of information areas in the form of pits or hills, a different geometrical depth, or height, and/or a different angle of inclination of the walls of the areas will be selected because this is simpler to realize.

In the first embodiment of the record carrier in accordance with the invention the phase depth of the first type of information areas, observed with a first read-beam component, is equal to that of the second type of information areas, observed with the second read-beam component. When such a record carrier is read only one read method is used, either the central aperture method or the differential method.

It is alternatively possible to read one type of information pit or hill with the central aperture method and the other type with the differential read method. In a record carrier which is suitable 115 for this the first type of information areas, observed with the first read-beam component, have a first phase depth which differs from a second phase depth of the second type of information areas, observed with the second read-beam component.

Preferably, the first phase depth is then approximately 1101 and the second phase depth approximately 180".

It is not absolutely necessary that the two 125 types of information areas have different dimensions. The different geometries for the two types of information areas may be, and are preferably, realised by making the orlentations of the information areas different. In the preferred 130 embodiment of a record carrier in accordance with the invention, in which the two types of information areas have the same dimensions, the longitudinal direction of the first type of information areas is transverse to that of the second type.

The information areas of the first type are read with a read-beam component which is polarized in a first direction, for example the longitudinal direction of these areas, and the information areas of the second type with a read beam component which is polarized in a second direction, transverse to the first direction. In such a structure, a "fishbone" structure, the longitudinal directions of the information areas make an angle of for example 450 with the track directions and a maximum information density is obtained. In an information structure with information areas of uniform dimensions not only may digital information be stored, but also analog information. In the last-mentioned case the information is encoded in the frequency of and/or the distance between the information areas.

For a disk-shaped record carrier the adjacent track portions may comprise information areas of the first type and information areas of the second type respectively. Preferably, the information structure then comprises two spiral tracks of which the first and the second track respectively comprise information areas of the first and the second type, the track revolutions of the first spiral track being situated between those of the second spiral track. When this record carrier is read one spiral track is scanned completely and subsequently the second spiral track.

It is alternatively possible that consecutive track portions within one track revolution differ from each other in that they comprise information areas of the first type and information areas of the second type respectively. This information structure is attractive if the two said read methods are to be used.

In a third embodiment of a record carrier in accordance with the invention, but which is provided with two information layers, a first information layer comprises only information areas of a first type and the second information layer only information areas of the second type.

It has already been proposed, inter alla in United States Patent Specification No. 3,853,426, to increase the information content of an optically readable record carrier by arranging two information layers at different levels in the record carrier body. In order to avoid crosstalk from the other information layer when reading out one layer, the information layers should be spaced at a mutual distance which is large relative to the depth of focus of the read objective.

This gives rise to the problem that the read beam should be focussed through a comparatively thick layer, so that the aberrations of the read objective will play a part. Moreover, the focussing of the read objective must be readjusted upon each transition from the one information layer to the second layer. However, 4 GB 2 058 434 A 4 i 35 when the first information layer comprises a first type of information areas and the second information layer a second type of information are- as and a first and a second read-beam component with mutually perpendicular directions of polarization are employed for reading, in such a way that the first type of information areas produce a maximum modulation in the first read-beam component and are virtually not observed by the second read beam component, whilst the second type of information areas produce a maximum modulation in the second read-beam component and are hardly observed by the first read-beam component, the two information layers may be situated close to each other, namely within the depth of focus of the read objective and can yet be read separately.

The track portions of the first information layer may be situated above those of the second information layer. An even better read-out separation of the two information layers is obtained if the track portions of the first information layer are situated between those of the second information layer.

In a record carrier with two information layers each information layer comprises two types of information areas, those track portions of the two information layers which comprise the same type of information areas being interposed between one another. For this record carrier the information density may be four times as great as that of known record carrier with only one type of information areas.

The invention cannot only be used in a record carrier which is completely provided with information, but also in a record carrier in which the user himself can write information. In such a record carrier, which is described inter alla in British Patent Application No. 2,01 6,744A an optically detectable so- called servo track, is provided. This servo track comprises sector addresses whose number is constant, for example 128, per track revolution. These sector addresses cover only a small part of the servo track. The record carrier portions between the sector addresses are provided with an inscribable material, for example a thin metal layer, in which the user can write the desired information by means of a laser beam, by locally melting the metal. A sector address inter alla contains address information relating to the associated inscribable record carrier portion in the form of address information areas which are spaced from each other by intermediate areas. In accordance with the invention the information areas of two adjacent sector addresses may have mutually perpendicular longitudinal directions. As a result of this the information density in this type of record carrier can also be increased. In a record carrier portion which corresponds to a specific sector address information can be written in the information areas having the same orientation as the address information areas in the sector address.

The invention may also be applied to an inscribable record carrier in which the information areas of all sector addresses have the same orientation and the same dimensions. It is then possible that in a "blank" portion of the record carrier corresponding to a specific sector address two information tracks are recorded by the user. If such a record carrier has been inscribed with information which is useful to a specific user it may be characterized in that there is provided an optically detectable servo track which includes sector addresses, the information associated with a specific sector address being contained in two information tracks of which at least one information track is shifted relative to the servotrack and transverse to the track direction, and the longitudinal direction of the information areas in one information track being transverse to that of the information areas in the second information track.

The invention also provides an apparatus for reading the record carrier, which apparatus is provided with an optical read system comprising a radiation source producing a read beam, an objective system for focussing the read beam to a read spot on the information structure, and a radiation-sensitive detection system for converting the read beam which has been modulated by the information structure into an electrical signal, wherein at the location of the information structure the read beam produced by the optical read system comprises two read beam components, which may be present simultaneously or not, with mutually perpendicular directions of polarization, which are respectively parallel and perpendicular to the longitudinal direction of one type of information areas.

It is to be noted that in the German Patent Application No. 2,634,243, which has been laid open to public inspection, a combined write-read apparatus is described, in which two radiation beams with mutually perpendicular directions of polarization are incident on the record carrier.

However, these beams are used for simultaneously writing two tracks and for enabling either two tracks to be scanned simultaneously or a tracking signal to be generated during read-out. The record carrier then contains only one type of information area and the directions of polarization of the two beams are not parallel to or perpendicular to the longitudinal direction of the information areas.

in the read apparatus in accordance with the invention only that read beam component maybe present at the location of the information structure which corresponds to the type of information area being read instantaneously. In such an apparatus the direction of polarization of the read beam should be changed each time the other type of information area is to be read. For this purpose, it is for example possible to arrange a half-wave plate between the radiation source and the objective system, which plate can be moved into and out of the read beam. It is also GB 2 058 434 A 5 possible that the radiation source, in the form of semiconductor diode laser, is mounted for rotation through 900. Furthermore, there may also be arranged two diode lasers mounted on a common movable support, which produce read beams whose directions of polarization are perpendicular to each other. If the read apparatus comprises polarization-sensitive means for separating the read beam which has been modulated by the information structure from the unmodulated beam, a polarization rotator may be arranged between a polarization-sensitive beam splitter and the objective system, which rotator alternately rotates the direction of polarization of -15 both the read beam emitted by the radiation source, which direction makes an angle of 451 with the longitudinal direction of one type of information areas, and of the read beam reflected by the information structure through an angle of approximately +451 and an angle of approximately -451.

Steps may also be taken to ensure that the direction of polarization of the read beam at the location of the information structure always makes an angle of approximately 451 with the longitudinal direction of one type of information area. The read beam may then be considered resolved into a beam component with a direction of polarization parallel to and a beam component with a direction of polarization perpendicular to the longitudinal direction of one type of information area. The detection system should then be polarization-sensitive in order to enable the information in the two read beam components, which are both present continuously, to be processed separately. The detection system may then comprise one detector preceded by a rotatable polarization analyser, or a polarization-sensitive beam splitter and two detectors, or one polarization-insensitive beam splitter and two detectors which are each preceded by a polarization analyser.

For inscribing and reading the two types of information areas with substantially perpendicular longitudinal directions relative to each other a combined write-read apparatus may be employed, which has the characteristic features of the aforementioned read apparatus and which furthermore comprises a radiation source producing a write beam, an intensity modulator for switching the intensity of the write beam between the first (write) level and a second, lower, level. In such an apparatus the write spot formed on the information layer by the objective system may be elongate and means may be provided for positioning the write spot in two orientations in which the longitudinal directions of the write spot differ by substantially 901 from each other, whilst said longitudinal directions both make an angle of approximately 451 with the longitudinal direction of the servo track. The intensity modulator may be constituted by means for controlling the power supply of the radiation source.

Embodiments of the invention will now be described in more detail with reference to the 130 accompanying drawings, in which Figure 1 is an elevation of a small part of a record carrier in accordance with the invention, Figure 2 shows a part of a tangential cross- section of this record carrier, Figure 3a shows a part of a radial cross-section of a first embodiment of the record carrier, Figure 3b shows a part of a radial cross-section of a second embodiment of the record carrier, Figure 4 shows a radial cross-section of a small part of a third embodiment of the record carrier.

Figure 5 shows an elevation of a record carrier in which the longitudinal directions of the two types of information areas are transverse to each other, Figure 6 is an elevation of a record carrier with two spiral tracks, Figure 7 is a view of a part of a record carrier containing different types of information areas per track, Figure 8 is a tangential cross-section of a part of this record carrier, Figure 9 is a radial cross-section of a part of a record carrier with two information layers, Figure 10 is a radial cross-section of a record carrier with two information layers which each contain two types of information areas, Figure 11 shows a first embodiment of a read apparatus, Figure 12 represents cross-sections, in the far field of the information structure, of the zero-order beam and the first-order beams formed by the information structure, Figure 13 represents the variation of the amplitude of the information signal as a function of the phase depth, Figure 14 represents the variation of the phase difference produced in a read beam bycontinuous groove as a function of the width of said groove, and for different directions of polarization, Figure 15 represents the variation of the phase difference produced in a read beam by a continuous groove as a function of the depth of said groove and for different directions of polarization, Figure 16 is an elevation of a record carrier in which information can be written by a user, Figure 17 is an elevation of a part of a record carrier inscribed by a user, Figure 18 shows a second embodiment of a read apparatus, Figure 19 shows a first embodiment of a polarization-sensitive detection system for the read apparatus, Figure 20 shows a second embodiment of such a detection system, Figure 21 schematically represents a first embodiment of a combined write/read apparatus, and Figure 22 schematically represents a second embodiment of such an apparatus.

As is shown in Figure 1 the information structure comprises a plurality of information areas 4(41, which are arranged in accordance 6 GB 2 058 434 A 6 with tracks 2(21). The areas 4(41) are spaced from each other in the track direction, or tangential direction t, by intermediate areas 5. The tracks 20) are spaced from each other in the radial direction r by narrow lands 3.

The information areas 4(41) may comprise pits pressed in the record carrier surface or hills projecting from said surface. In the case of central-aperture reading i.e. if the information carrier should have a greater phase depth, the information areas will preferably be pits.

The information to be disseminated by means of the record carrier is contained in the variation of the areas structure in the tangential direction only. If a colour television program is stored in the record carrier, the luminance signal may be encoded in the variation of the spatial frequency of the information areas 4(41) and the chrominance and audio signal in the variation of the lengths of said areas. The record carrier may alternatively contain digital information. In that case a specific combination of information areas 4(41 and intermediate areas 5 represent a specific combination of digital ones and zeros.

The record carrier can be read with an apparatus which is schematically represented in Figure 11. A monochromatic and linearly polarized beam 11 which is emitted by a gas laser 10, for example a helium-neon laser, is reflected to an objective system 14 by a mirror 13. The path of the radiation beam 11 includes an auxiliary lens 12, which ensures that the pupil of the objective system 14 is filled. In that case a diffraction-[! mited read spot V is formed on the information structure. The information structure is 100 schematically represented by the tracks 2(2l): thus, the record carrier is shown in radial crosssection.

The information structure may be located at the side of the record carrier which faces the laser. However, preferably, as is shown in Figure 11, the information structure is situated at the side of the record carrier which is remote from the laser, so that reading is effected through the transparent substrate 8 of the record carrier. The advantage of this is that the information structure is protected against fingerprints, dust particles and scratches.

The read beam 11 is reflected by the information structure and, as the record carrier is rotated by means of a platter 16 driven by a motor 15, it is modulated in accordance with the sequence of the information areas 4(4') and the intermediate areas 5 in a track being read. The modulated read beam again passes through the objective system 14 and is reflected by the mirror 13. In order to separate the modulated read beam from the unmodulated read beam a beam splitter 17 has been included in the radiation path. The beam splitter may be a semitransparent mirror, but alternatively may be a polarization-sensitive splitter prism. In the last-mentioned case a quarter-wave plate should be included between the objective system and the splitter prism. The quarter-wave is then a quarter of the wavelength of the read beam 11. The beam splitter 17 reflects a part of the modulated read beam to a radiation-sensitive detection system 19. In the case of the central-aperture read method this detection system comprises a single detector which is disposed on the optical axis of the read system. The output signal S, of this detector is proportional to the information being read. If use is made of the differential read method, the detection system comprises two tangentially shifted detectors, which are arranged in the far field of the information structure. Subtracting the output signals of the detectors from each other yields a signal which is modulated in accordance with the information being read.

The information structure is illuminated with a read spot V whose dimension is of the order of magnitude of that of the information areas 4(41). The information structure may be regarded as a diffraction grating which splits the read beam into an undiffracted zero spectral order subbeam, a plurality of first spectral order subbeams and a plurality of higher spectral order subbeams. For read-out, it is mainly the subbeams which are diffracted in the longitudinal direction of the tracks which are of interest, and of these beams specifically the subbeams which are diffracted into the first orders. The numerical aperture of the objective system and the wavelength of the read beam have been adapted to the information structure in such a way that the higher-order subbeams for the most part fall outside the pupil of the objective system and are not incident on the detector. Moreover, the amplitudes of the higher-order subbeams are small relative to the amplitudes of the zero-order subbeam and the first-order subbeams.

Figure 12 represents the cross-sections of the first-order subbeams, which have been diffracted in the track direction; in the plane of the exit pupil of the objective system. The circle 20 with the centre 21 represents the exit pupil. This circle also represents the cross-section of the zero-order subbeam WO, 0). The circles 22 and 24 with the centres 23 and 25 respectively represent the cross-sections of the first-order subbeam bW, 0) and b(-1, 0) respectively. The arrow 26 represents the track direction. The distance between the centre 21 of the zero-order subbeam and the centres 23 and 25 of the first-order subbeams is determined by A/p, where p (compare Figure 1) represents the spatial period, at the location of the read spot V, of the areas 2, afid A the wavelength of the read beam.

From the method adopted for describing the read operation it follows that in the hatched areas in Figure 12 the first-order subbeams overlap the zero-order subbeam and that interference occurs.

The phases of the first-order subbeams vary if the read spot moves relative to an information track. As a result of this the intensity of the total radiation which traverses the exit pupil of the objective system will vary.

When the centre of the read spot coincides with the centre of an information areas 4(41) a r 7 specific phase difference 0, referred to as the phase depth, will exist between a first-order subbeam and the zero-order subbeam. If the read spot moves to the next area, the phase of the subbeam b(+ 1, 0) increases by 27t. It is therefore correct to state that when the read spot moves in the tangential direction the phase of said subbeam varies by o)t relative to the zero-order subbeam. Therein 6o is a time frequency which is determined by the spatial frequency of the information areas 2 and by the speed with which the read spot travels over a track. The phase 0(+ 1, 0) and 0(-11, 0) of the subbeam b(+ 1, 0) and of the subbeam b(-1, 0) respectively relative to the zero-order subbeam WO, 0) may be represented by:

and by 0(+1, o)=0+0)t 0(-1, M=0-0)t respectively.

When the portions of the first-order subbeams and the zero-order subbeams traversing the objective system are combined on one detector, as in the central-aperture read method, the time dependent signal of this detector may be represented by:

SCA=13(0). cos 0. cos o)t where B(O) decreases at decreasing values of 0. In the case of the differential read method two detectors 19' and 1911 represented by dashed lines in Figure 12, are arranged in the areas of overlap of the zero-order subbeam with the firstorder subbeams. The time-dependent different signal from these detectors may be represented 3 5 by:

S,j=13(0). sin 0. sin 6ot Figure 13 represents the variation of the amplitude A,=13(0). cos 0 and of the amplitude A2=13(0). sin 0 as a function of the phase depth 0 as calculated by the Applicant and corroborated by experiments. For 0=901 both A, and A2 are zero. A, reaches a maximum for 0=1 800. The maximum for A2 is situated at approximately 1101. The depth of an amplitude structure may therefore be said to equal 7r.

The values of the phase depth 0 at which for 105 the two read methods a maximum destructive and constructive intMerence occurs between the first-order subbeams and the zero-order subbeam respectively, i.e. a maximum and minimum modulation of the detector signal is obtained, are 110 given in the following Table:

Destructive Interference CA. Read-out D.L Read-out 8 2 O= (M+ 1)7t GB 2 058 434 A 7 717 Constructive Interference CA. Read-out D.L Read-out 7t 717 O= (- +m-) 2 2 O= (- +M7t) 2 Here m represents an integer. This Table is valid if no powerful subbeams of an order higher than one enter the pupil of the read objective.

The phase depth observed by a read beam depends on the geometry of the information areas, specifically the geometrical depth of an information pit, or the geometrical height of an information hill, and on the angle of inclination of the walls of the information areas. Specifically the phase depth also depends on the effective wavelength of the read beam relative to the width b of the information areas, in the plane of the intermediate areas and the lands 3. If the effective wavelength is of the same order of magnitude as or greater than the width b of the information areas, the state of polarization of the read beam will have a substantial influence on the phase depth. The direction of polarization of the read beam will already play a part at an effective wavelength which is approximately 1.5 times the effective width W,,ff) of the information areas. The widt h b and the effective width W,,ff) are represented in Figure 3a.

The influence of the state of polarization on the phase depth 0 is illustrated by Figure 14, which represents the theoretical variation of the phase (p of the local electromagnetic field at the bottom relative to the field at the top of a continuous groove g as a function of the width b of the groove, expressed in the effective wavelength A.. The groove g, which is also shown in Figure 14 has a depth of 0.24 A, The curves P,, and P_L represent the variation of the relative phase (p for parallel and perpendicularly polarized radiation respectively, whilst the straight line PS represents the variation of the relative phase (p as predected by scalar diffraction theory, in which no allowance is made for the direction of polarization of the radiation. Figure 14 shows that as soon the width of the groove g becomes of the order of magnitude of the effective wavelength, the phase (p for the various directions of polarization becomes different. According as the width b decreases relative to the effective wavelength, the curves P,, and P.L will deviate more from each other and from PS.

Figure 15, for a specific width, b=0.64 Ae, represents the variation of the relative phase (p as a function of the depth d, expressed in A,, for the various directions of polarization by means of the curves Q,, and Q.L, Q, represents the variation of the relative phase (p as predicted by scalar diffraction theory. There is a direct relationship between the relative phase (p shown in the Figures 14 and 15 and the phase depth (p defined in the foregoing: if (p increases from 0 to 7r12 rad. the phase depth 9) will increase from to 2t rad. This applies strictly to scalar diffraction theory and in approximation to vectorial diffraction theory. From Figure 15 it is apparent that the phase 8 GB 2 058 434 A 8 depth of 7r rad. required for optimum centralaperture reading, which corresponds to a relative phase (p=7r12 rad. is reached at a groove depth of approximately 0.20 A. in the case of perpendicularly polarized radiation. For this groove depth the phase depth for parallel polarized radiation is approximately 7C - rad., 2 so that the groove is hardly observed with this radiation and in the central-aperture mode. For an optimum read-out of the groove with parallelpolarized radiation in accordance with the centralaperture method the groove depth should be approximately 0.4 A.. For this groove depth the phase depth for perpendicularly polarized radiation is approximately 1.6 7r rad.

It is to be noted that Figures 14 and 15 apply to a continuous groove. For tracks comprising information areas the relative phase (p will exhibit a similar variation for the various directions of polarization.

The effect illustrated with Figures 14 and 15 is employed in order to increase the information density. Depending on the wavelength of the read beam to be used, the width of the information areas is selected so that the requirement is met that A.ff is greater than or substantially equal to b.ff' If a He-Ne laser beam with a wavelength A.=633 nm is used and the information is read through a substrate with a refractive index n=1.5, 95 the track width should at the most be of the order of 420 nm. A. is the wavelength in free space. The record carrier can also be read by means of a beam produced by a semiconductor diode laser, such an A1GaAs laser whose wavelength may be between 780 rim and 860 nm. When such a beam is used, in the case of read-out through a substrate for which n=1.5, the track width should at the most be of the order of 520 rim to 570 nm.

Moreover, care is taken that all information areas are elongate, i.e. that their length is at least of the order of one and a half times their width, because only for this type of information areas is a difference in phase depth obtained between perpendicularly polarized radiation and parallelpolarized radiation. Preferably, the length of the information areas is at least twice the effective wavelength.

Furthermore, of two adjacent track portions the information areas of the one track portion are optimized for read-out with perpendicularly polarized radiation and the information areas of the second track portion for read-out with parallel-polarized radiation. As has been demonstrated by means of Figures 14 and 15 this 120 optimization can be achieved by adapting the geometrical depths of the information areas.

In Figures 14 and 15 it has been assumed that the groove g has perpendicular walls. However, in practice the walls of the information areas will 125 have an angle of inclination which differs from zero degrees owing to the methods of recording and copying used in the manufacture of the record carrier.

As is described in the article:---Laserbeam recording of video-master disks- in: -Applied Optics" Vol. 17, No. 3, pages 2001-2006, the information is inscribed in a so-called master by exposing a photo-resist layer on a substrate to a laser beam whose intensity in modulated in accordance with the information to be written. After the exposure the photo-resist is developed, yielding a pit structure or a hill structure. Merely, because of the intensity distribution of the write beam used, the ultimate record carrier will have oblique walls. The developing process also affects the wall steepness; the wall steepness increases as the developing time increases. From the developed master, mother disks are made in known manner, from which in turn matrices are made. By means of the matrices a large number of record carriers can be pressed. In order to facilitate separation of the replicas from the matrix, the angle of inclination of the wall should preferably be as large as possible. In order to obtain a desired effective depth of the information areas or height of the information hills, the geometrical depth or height should be greater than in the case of information areas with perpendicular walls.

Figure 2 shows a small part of the tangential cross-section of the record carrier of Figure 1, whilst Figure 3a shows a part of this record carrier in radial cross-section. The information structure may be covered with a layer 6 of a highly reflective material, such as silver or aluminium or titanium. It is to be noted that the polarization effects are stronger according as the optical conductivity of the layer 6, which is givan by the extinction coefficient of this layer, increases. On the layer 6 a protective layer 7 may be deposited, which protects the information structure against mechanical damage such as scratches. Furthermore, the tangential angle of inclination Ot and the radial angle of inclination Or are given in Figures 2 and 3a. These angles of inclination are of the same order of magnitude.

The desired difference between the effective depths of the two types of information areas 4 and 4' can be realized, as is indicated in Figure 3c, by selecting the geometrical depths d, and d2 Soas to be different. The areas 4 are then intended to be read with parallel-polarized radiation, whilst the areas 4' are intended to be read with pdrpendicularly polarized radiation.

The desired difference between the effective depths, as is shown in Figure 3b, can also be realized by making the radial angle of inclination 01 of the information areas 4 smaller than the radial angle of inclination 02 of the information areas 41. In an embodiment of a record carrier in accordance with Figure 3b which is adapted to be read entirely in the central aperture mode and in which the information areas are pits with a depth of approximately 220 nm and a width b of approximately 375 nm, the angle of inclination 01 h GB 2 058 434 A 9 is approximately 250 and the angle of inclination.

02 approximately 5 5 1. The refractive index n of the protective layer 8 is 1.5 and the layer 6 is a silver layer. This record carrier is intended to be read with a read wave length of 820 rim and via a read objective with a numerical aperture of 0.58.

In an embodiment of a record carrier in accordance with Figure 3b, which is to be read entirely in the differential mode and in which the information areas are hills with a height of 75 approximately 150 nm and a width b of approximately 625 nm, the angle of inclination of the hills, which are read with parallel-polarized radiation is approximately 570 and the angle of inclination of the hills which are read with 80 perpendicularly polarized radiation is approximately 2 5 0. In this record carrier the layer 6 is also a silver layer and the refractive index n of the protective layer 8 is 1.5. The read wavelength is again 820 nm and the numerical aperture of the 85 read objective is 0.54.

Obviously, it is also possible that both the geometrical depths and the angles of inclination of the information areas 4 and 4' are different.

The information areas shown in Figures 3a and 90 3b have been optimized for one read method.

However, the information areas 4 may be optimized for central-aperture reading and the information areas 4' for differential reading.

Figure 4 shows a radial cross-section of a small part of a record carrier designed for this purpose.

The information areas 4', which should yield a phase depth (p=l 101, are now so shallow that they have a V-shaped structure.

A record carrier with two types of information pits which have been optimized for reading by means of a perpendicularly polarized read beam and a parallel polarized read beam respectively, may also be adapted so that it is entirely readable by means of the differential method. In that case the radial cross-sections of both the information pits 4 and of the information pits 41 will be V- shaped. The difference between the effective depths of the information areas 4 and 4' is then solely determined by the radial angles of inclination of said information areas.

The information areas cannot only be distinguished in respect of their dimensions but also in respect of their orientations. Figure 5 is a plan view of a small part of such a record carrier.

The information areas 4 and the information areas 4' all have the same dimensions, even in their longitudinal directions 14 and 'C. The longitudinal direction 14 of the information areas 4 make an angle a, which is preferably 900, with the 120 longitudinal directions 14,of the information areas 4'. In an information structure with this type of information areas a digital signal may be stored, a specific combination of information areas 4 and 4' and intermediate areas 5 representing a specific combination of digital zeros and ones. The information areas 4 and 4' in accordance with Figure 5 may also be used for the storage of analog information. The information is then contained in the mutual distances between the information areas 4 and those between the information areas 41.

The information areas 4 are read with a read beam whose direction of polarization is transverse to their longitudinal direction 14. For the information areas 4' this read beam is parallelpolarized and these information areas are virtually not observed by said read beam.

In an embodiment of a record carrier in accordance with Figure 5 which is adapted to be read in the central aperture mode, the information areas are pits with a depth of approximately 220 nm, a width b of approximately 375 nm and an angle of inclination of the walls of approximately 550. The refractive index n of the layer 8 is 1.5 and the layer 6 is a silver layer. This recording is intended to be read by means of a wavelength of 820 nm and via a read objective with a numerical aperture of 0.58.

In a record carrier which is adapted to be read entirely in either the central aperture mode or the differential mode the laterally adjacent track portions have different types of information areas. Preferably, as is shown in Figure 6, such a record carrier comprises two spiral tracks, the turns 2 of the one spiral 30 being interposed between the turns 2' of the other spiral 30'. When the spiral 30' is read the optical read head is for example moved from the inner edge of the record carrier towards the outer edge. After the last turn of this spiral has been read, the direction of rotation of the motor, by means of which the record carrier is driven, is reversed and the read head is moved over the record carrier from the outer edge to the inner edge, so that the spiral 30 is scanned in the reverse direction.

When a record carrier is read of which one type of information areas has been optimized for central aperture reading and the second type of information areas for differential reading, the two detectors by means of which the differential information signal is determined may also be employed to obtain the central aperture information signal. In the lastmentioned case the output signals of the two detectors are added. The detectors are then connected to an electronic circuit, in which the detector signals are combined additively in first time intervals and subtractively in second time intervals, and in which the resulting signals are further processed and rendered suitable for reproduction for example with a video apparatus or an audio apparatus. The transfer function of the system in which the detector signals are added differs slightly from the system in which the detector signals are subtracted from each other. If the information is stored in digitized form, the change in transfer function when changing from one track to a next track, will not be perceptible in the signal which is ultimately delivered by the read apparatus. If the information is recorded in a different manner, for example in the form of a frequency- modulated signal, the change-over between the transfer functions may become perceptible. The one transfer function will for GB 2 058 434 A 10 example cause different grey shades or a different saturation in the television picture than the other transfer function. In the case of an audio signal the change-over between the transfer functions may become audible as an undesired frequency.

If a television program is stored in a record carrier in which one television picture per revolution is written, flicker at a frequency of 12.5 Hz will occur at a speed of rotation of 25 revolution/sec, as a result of the variation in the grey shades or in the colour saturation. Flicker of this frequency is perceptible to the human eye and is therefore annoying.

In order to render this effect invisible the 80 information areas of consecutive track portions within one track may be made different. Figure 7 shows a part of such an embodiment of a record carrier. This Figure shows a larger part of the record carrier than Figure 1, so that the individual information areas can no longer be distinguished. The information tracks have been divided into portions a, which comprise information areas which are read with a first direction of polarization in the differential mode, and portions b, which comprise information areas which are read with the second direction of polarization in the central aperture mode. Preferably, the perpendicularly polarized beam is used in the central-aperture mode and the parallel-polarized beam is the differential mode.

Figure 8 is a tangential cross-section of a part of the record carrier of Figure 7, at the location of the transition from a track portion a to a track portion b. After the foregoing this figure is selfexplanatory.

In the case of a television program the track portions a and b each contain the information of one television line. If the television picture comprises 625 lines, switching between the one read system and the other is effected at a frequency of the order of 7.5 kHz. A flicker effect of this high frequency is no longer visible.

In order to enable a correctly timed change- over from addition of the detector signals to subtraction of these signals and vice versa during read out of the record carrier, the record carrier may contain a pilot signal at the location of the transitions between the track portions a and b.

Such a pilot signal may also be recorded on a record carrier containing an audio program.

If a television signal has been recorded, the field synchronizing impulses or the picture synchronizing pulses may be used as switching signal, so no separate pilot signal is needed.

Figure 9 in radial cross-section shows a small part of a record carrier comprising two information layers 31 and 31 '. The information layer 3 1 comprises a first type of information areas 4 and the information layer 311 a second type of information areas 4'. Either the angles of inclination (0, and 04) or the depths (d 1 and c14) or, as is shown in Figure 9, both the angles of inclination and the depths of the areas 4 and 4', which may again be pits or hills, may then differ. It 130 is alternatively possible that the information areas 4 and 4' all have the same dimensions and that the longitudinal directions of the areas 4 are transverse to those of the areas 4'.

The track portions of the information layer 31 may be situated straight above those of the information layer 31 '. As is shown in Figure 9, the track portions of the one information layer are preferably situated adjacent those of the other information layer.

In a record carrier with two information layers these layers may alternatively each contain twa types of information areas. A radial cross-section of a small part of such a record carrier is shown in Figure 10. In each of the information layers the.

track period may then be reduced by for example a factor two, so that the total information content is for example a factor four greater than the information content of a known record carrier with only one information layer and one type of information areas. The track portions of the first information layer which comprise information areas of a first type, shouldthen be interposed between the track portions of the second information layer which comprise information areas of the same type, as is shown in Figure 10.

It is to be noted that in this figure, as well as in the preceding figures, the dimensions of the information areas have been exaggerated relative to, for example, the thickness of the substrate 8 for the sake of clarity.

In for example the previous Netherlands Patent Application No. 78 02859 (PHN 9062) it has already been proposed to employ an optical record carrier as a storage medium for information other than video information, and specifically as a storage medium in which information can be written by the user himself. Examples of this is information supplied by an (office) computer or radiograms in a hospital. For this application the user is supplied with a record carrier which is provided with servo track, which may be spiral, which extends over the entire record carrier area.

During the recording of the information by the user the radial position of the write spot relative to the servo track is detected and corrected with the aid of an opto-electronic servo system, so that the information is written with great accuracy ip a spiral track of constant pitch, or in concentric. tracks with a constant track distance. The servo track is divided into a large number of sectors, lior example 148 per track revolution.

Figure 16 shows such a record carrier 40. The concentric servo tracks are designated 41 and the sectors are designated 42. Each sector comprises a track portion 44, in which information may be written, and a sector address 43, in which in addition to other control information the address of the associated track portion 44 is encoded in for example digital form in address information areas 45. The address information areas are spaced from each other in the track direction by intermediate areas 46. The address information areas are preferably pits pressed into the record 11 GB 2 058 434 A 11 carrier surface or hills projecting from this surface.

In accordance with the invention, as is shown in the inset of Figure 16, the longitudinal directions of the address information areas 45 and 45' of two adjacent sector addresses are perpendicular or substantially perpendicular to each other and these areas have uniform dimensions. These dimensions have been selected relative to the wavelength of the read beam so that they produce a maximum modulation in a read beam component with a direction of polarization parallel to their longitudinal direction and at the same time are hardly observed by a read beam component with a direction of polarization transverse to their 80 longitudinal direction. The two servo track portions with mutually perpendicularly oriented address information areas may be arranged close to each other when two mutually perp andicularly polarized read beam components are used, so that the information density can be very high.

It is then necessary that the information areas which are written into two adjacent blank track portions 44 are distinct from each other, for example in respect of their orlentations. In the inset of Figure 16 these information areas 47 and 47' are shown dashed.

For the sake of clarity the width of the tracks 41 in Figure 16 has been exaggerated relative to the lengths of the sectors 42.

The invention may also be employed for the recording of information by the user. In the record carrier as supplied to the user the address information areas in the sector addresses will for example all have the same orientation and dimensions. The information recorded by the user will now be distributed over two tracks which are for example situated on both sides of the servo track, the longitudinal direction of information areas in the first information track being transverse to that of the information areas in the second track. Figure 17 shows a small part of such a record carrier inscribed by the user.

The sector addresses 43 of the tracks 41 comprise address information areas 48. Each sector address 43 is associated with a specific information block. The information of such a block is divided over two information portions 50 and 50'. The longitudinal direction of the information areas 47 in the information track portion 50 is 115 transverse to that of the information areas 47' of the information track portion 50'.

It is alternatively possible that one of the information track portions 50 and 501 coincides with a blank track portion 44.

The information recorded in the two information track portions 50 and 50' need not belong to one block of information, but may be of different sorts and for example form part of two different programs.

In an embodiment of a record carrier which has been inscribed by the user, in which the information layer is a metal layer and the informations areas 47 and 47' comprise pits melted into said layer, the width of the information areas 47 and 47' is approximately 270 rim. These areas are written and read with a diode laser beam having a wavelength of 820 rim, with an objective having a numerical aperture of approximately 0.58 and via a substrate having a refractive index n=1.5. A width of the information areas between 200 and 400 rim then still yields acceptable results.

The read beam components with mutually perpendicular directions of polarization required for reading the record carrier can be obtained in different manners. As is shown in Figure 11, a plate 33, which is pivotable about an axis 36, may be included in the radiation path before a polarization-insensitive beam splitter 17. Said plate may comprise two parts 34 and 35, the part 34 being of a birefringent material and constituting a half-wave plate for the radiation used, whereas the part 35 is for example of glass.

The source 10 emits linearly polarized radiation, whose direction of polarization is for example parallel to the longitudinal direction of the information areas on the record carrier. When the part 34 of the plate 33 is situated in the radiation path, the direction of polarization of the beam 11 is unchanged, and this beam is suitable for reading one type of information areas. If the part 35 of the plate 33 is situated in the radiation path, the direction of polarization of the read beam 11 is rotated through 900 and this beam is suitable for reading the second type of information areas.

The plate 33 is preferably situated at the location of the smallest constriction of the beam 11. It may also be interposed between the auxiliary lens 12 and the radiation source 10.

The plate 33 may also be employed if the socalled feedback effect is employed when reading is effected by means of a diode laser as radiation source. Use is then made of the fact that when the radiation beam emitted by the diode laser is reflected to the diode laser by the record carrier, the intensity of the emitted laser beam and the electrical resistance of the diode laser will increase. When an information track of the record carrier is scanned with such a laser beam the said intensity and electrical resistance will vary in accordance with the sequence of information areas in the relevant track. The record carrier may then be read by for example detecting the intensity variations of the laser beam by means of a photodiode at the rear of the diode laser. In that case no beam splitter is necessary in order to separate the incident and the reflected beam from each other.

Also when in a read apparatus employing the feedback effect the half-wave plate is included in the beam, the direction of polarization of the read beam received by the diode laser will be the same as that of the beam emitted by the diode laser, because said plate is traversed twice.

The two beam components with mutually perpendicular directions of polarization can also be obtained by mounting the laser source on a support which is rotatable between two positions which are approximately 900 apart. In this case GB 2 058 434 A 12 the use of a semiconductor diode laser as radiation source is to be preferred. It is alternatively possible to employ two diode lasers, which emit radiation beams whose directions of polarization are transverse to each other. These lasers may be mounted on a common support. By rotating this support the direction of polarization of the radiation which is projected onto the information structure may be changed.

The signals for rotating the plate 33 or the laser source may be derived from the signal read from the record carrier. Said record carrier is then provided with marks which indicate when the direction of polarization of the read beam is to be changed.

The methods described in the foregoing for obtaining two mutually perpendicularly polarized beam components cannot be used in a read apparatus which already includes polarization sensitive elements. In that case use can be made of the solution illustrated in Figure 18. In this figure a polarization-sensitive splitter prism is designated 1V, which prism is employed for separating the read beam which has been modulated by the information structure from the 90 beam emitted by the source. The radiation source is a diode laser which emits a linearly polarized beam, whose direction of polarization makes an angle of 451 with the longitudinal direction of one type of information areas on the record carrier.

The radiation path includes a polarization rotator 37 behind the prism 1P, which rotator is capable of rotating the direction of polarization of both the beam emitted by the radiation source 10 and that of the beam reflected by the information structure 100 through 450 anticlockwise or through 45 0 clockwise. The polarization rotator may be Faraday rotator. By means of this rotator the direction of polarization of the read beam can be switched between parallel and transverse to the longitudinal direction of the information areas and vice versa, or, for reading the record carrier in accordance with Figure 5, between parallel to the longitudinal direction of the first type of information areas and parallel to the longitudinal direction of the second type of information areas.

The polarization rotator 37 also ensures that the read beam which has been modulated by the information structure has a direction of polarization which is transverse to that of the beam emitted by the source, so that the first mentioned beam is reflected out of the radiation path and to the detector 19 by the polarization sensitive prism 171.

In the embodiment of the read apparatus 120 discussed so far the record carrier is always read with radiation having only one direction of polarization, and all the radiation produced by the radiation source is utilized for the read-out.

In a further embodiment of the read apparatus the direction of polarization of the read beam at the location of the information structure makes an angle of 450 with the longitudinal direction of the information areas. This beam may be regarded as comprising two beam components, of which the first component has a direction of polarization which is parallel to the longitudinal direction of the information areas and the second a direction of polarization which is transverse to the said longitudinal direction. In such an apparatus the radiation-sensitive detection system should be polarization-sensitive.

For this purpose, as is shown in Figure 19, a rotatable polarization analyser 38, whose direction of transmission is designated 39, may be included before a single detector 19. Figure 19 shows only the part of the read apparatus near s the detector 19. For the remainder the apparatus is similar to that of Figure 11, the plate 33 being dispensed with. In Figure 19 the beam components with mutually perpendicular directions of polarization are represented by the solid lines 11' and the dashed lines 1111 respectively. In reality the beams 11 ' and 11 coincide. In the position of the analyser shown the beam 11 1 is transmitted to the detector and a first type of information areas can be read. If the analyser is rotated through 901 the beam 11 " is transmitted and the second type of information areas can be read.

As is shown in Figure 20, the polarisationsensitive detection system may also be constitited by a polarization-sensitive splitter prism 40 and two detectors 19' and 19". The prism 40 transmits the beam 11 ' with a first direction of polarization to the detector 19' and reflects the beam 11 " with a second direction of polarization, transverse to the first direction of polarization, to the detector 1 C. The output signal Si' of the detector 19' represents the information which is stored in the first type of information areas and the output signal SM of the detector 1W' the information stored in he second type of information areas. In an electronic circuit, not shown, for processing the detector signals provisions are then made for alternately utilising the signal Si' and the signal Si".

In a further embodiment of a polarizationsensitive detection system the polarization- sensitive prism 40 of Figure 20 are replaced by a neutral beam splitter and a polarization analyser is provided for each of the detectors 191 and 1 W. The directions of transmission of the two analysers are transverse to each other.

It is conceivable that the directions of polarization of the read beam components are not exactly parallel to and transverse to the longitudinal direction of one type of information aea, i.e. for the read directions indicated in Figures 19 and 20 that the directions of polarization of the read beam do not make an exact angle of 450 with the track direction. This enables the signal of one of the read beam components to be increased relative to the signal of the other read beam component. In this way the tolerances for one type of information area, if these are more difficult to realize, may be increased. The said deviation in the directions of polarization could be of the order of 20% to 30%.

Equipment by means of which the information IN 13 GB 2 058 434 A 13 -15 can both be written and read is known. For example in the previous Netherlands Patent Application No. 78 02859 (PHN 9062), which is incorporated herein by reference, a combined write-read apparatus is described, in which the write beam and the read beam are produced by the same radiation source. In this apparatus the intensity of the beam produced by the radiation source is switched between a first (write) level and a second level, for example by means of an intensity modulator, which second level is sufficiently high to read information but not high enough to record information. In Netherlands Patent Application No. 74 02289, which has been laid open to publis inspection, a write apparatus is described, in which a read spot is projected on the information layer at a short distance behind a write spot. By means of this read spot it is possible to check whether the information just recorded corresponds to the information intended to be written.

Figure 21 shows those elements of a combined write-read apparatus which are relevant for the present invention. As radiation source a gas laser 60 is used, for example a HeNe laser. The intensity of the laser beam 61 is varied with the aid of an intensity modulator 62, for example an acousto-optical modulator or an electro-optical modulator, which is controlled by a control circuit 63. The laser beam is reflected to the objective system 65 by a rotatable mirror 64, which objective system focuses the beam to a radiation spot V in the information plane, represented by the servo tracks 41 of the record carrier 1.

An elongate write spot, whose longitudinal direction is adjustable can be obtained by including in the radiation path, preferably as closely as possible to the entrance pupil of the 6bjective system 65, a rotatable diaphragm 66 with an aperture slit 67. If the diaphragm is not included, the beam 61 completely fills the pupil of the objective system and a diffraction- limited circular radiation spot is formed on the information layer. If the diaphragm is included in the radiation path, the beam 61 is completely transmitted in one direction, namely the direction of the aperture slit 67, and is stopped for the most part in a direction transverse thereto. The pupil of the objective system 65 is then no longer filled in an optimum manner. The write spot is then an elongate spot whose longitudinal direction is transverse to the longitudinal direction of the aperture slit. If the aperture slit 67 had the illustrative position shown in Figure 2 1, the longitudinal direction of the elongate spot would 120 coincide with the track direction. In reality, the aperture slit may assume two orientations, namely at angle of +450 and -450 relative to the position shown in Figure 2 1, so that the longitudinal direction of the write spot can make 125 angles of +451 and -450 with the longitudinal direction of the tracks.

During reading the diaphragm is removed from the radiation path, preferably as indicated by the arrow 68 in Figure 21, so thatthe read spot is 130 again a circular radiation spot.

The elongate write spot with adjustable orientation can also be obtained by a rotatable cylindrical lens instead of with a rotatable diaphragm.

The diaphragm or the cylindrical lens may also be used in an apparatus employing a diode laser as radiation source. Such a diode laser is designated 70 in Figure 22. The intensity of the beam produced by the diode laser can be controlled by varying the electric current through the electrodes 71 on the diode laser 70. The electric current is supplied by a current source 74, which is controlled by means of a control circuit 63. In many cases a diode laser produces an astigmatic beam, i.e. a beam having a crosssection which is greater in a first direction, for example by a factor two, than in a direction transverse to the first direction. If the pupil of the objective system is to be filled completely by means of a diode laser, an additional element, for example a cylindrical lens, should be included in the radiation path for correcting the astigmatism. However, in the combined write-read apparatus effective use can be made of the astigmatism of the diode laser. By passing the diode-laser beam through the objective system without correction an elongate radiation spot is obtained. The orientation of this spot relative to the tracks can be adjusted by rotating the diode laser 70. For this purpose said laser may be mounted on a holder 72, which is rotatable about an axis 73.

In the embodiments of the write-read apparatus described so far the polarization components required during read-out are obtained in one of the manners described with reference to Figures 11, 18, 19 and 20.

If, by means of the apparatus, two information track portions (50 and 50' in Figure 17) are to be written or read near each sector address, the radiation spot should be moved after the first information track portion has been written or read respectively in a direction transverse to the track direction over a distance equal to the width of the servo track plus the width of one information track portion (in the case of Figure 17) or over a distance equal to the width of the servo track, if one of the information track portions 50 or 50' coincides with the servo track. For this movement of the radiation spot use can be made of the pivotable mirror 64 which has already been provided in the apparatus for the purpose of tracking. During writing the servo track is used as reference for positioning the write spot. During reading both the servo track and an information track portion 50 or 501 may be used as reference.

During writing or reading of two information track portions 50 and 50' a sector 42 is scanned twice, one information track portion 50 or 50' being written or read during each scan.

When inscribing the two types of information areas whose longitudinal directions are transverse to each other, advantageous use can be made of an effect which is known per se. When an objective system with a high numerical aperture is 14 G B 2 0 5 8 434 A 14 used the distribution of the electric field energy within the radiation spot, which field energy ensures that pits are burnt into the information layer, is inherently astigmatic. In respect of the electric field energy the spot is larger in the direction of polarization of the radiation than in a direction transverse to the direction of polarization. When an objective system is used having a numerical aperture of 0.85 the length is 30% greater than the width. By suitably selecting the direction of polarization of the laser beam in the apparatus of Figure 22, the effective write spot can be made narrower by the use of said effect.

Claims

1. A record carrier containing information in an optically readable information structure, which comprises trackwise arranged information areas which in the track direction alternate with intermediate areas, laterally adjacent track 85 portions differing from each other in that they comprise information areas of a first type and information areas of a second type respectively, wherein all the information areas are elongate, the information areas of the first type have such a geometry that in a first read beam component, whose direction of polarization is parallel to the longitudinal direction of these information areas and whose effective wavelength is at least of the order of magnitude of the width of the information areas, they produce a maximum modulation and, moreover, in a second read beam component, whose direction of polarization is transverse to the longitudinal direction of the information areas and whose effective wavelength is equal to that of the first read-beam component, produce a mimimum modulation, and wherein the information areas of the second type have such a geometry that they produce a minimum modulation in the first read beam 105 component and moreover produce a maximum modulation in the second read-beam component.

2. A record carrier as claimed in Claim 1, in which the longitudinal directions of the two types of information areas coincide with the longitudinal direction of the tracks in which said information areas are situated, wherein the two types of information areas differ from each other in that at least one of the geometrical dimensions of said areas, which are not determined by the information stored, differ.

3. A record carrier as claimed in Claim 2, in which the information areas are situated outside the plane of the intermediate areas, and in which, for the first type of information areas, the distance 120 between the top of the information areas and the plane of the intermediate areas is greater than the corresponding distance for the second type of information areas.

4. A record carrier as claimed in Claim 2 or 3, 125 in which the information areas are situated outside the plane of the intermediate areas, and in which, for the first type of information areas, the angle of inclination between the walls of said areas and the normal to the record carrier is smaller than the corresponding angle of inclination of the second type of information areas.

5. A record carrier as claimed in Claim 2, wherein the phase depth of the first type of information areas, observed with the first read beam component, is equal to that of the second type of information areas, observed with the second read-beam component.

6. A record carrier as claimed in Claim 2, in which the information areas are disposed outside the plane of the intermediate areas, wherein the first type of information areas, observed with the first read-beam component, have a first phase d depth, which differs from a second phase depth corresponding to the second type of information areas, observed by means of the second read beam.

7. A record carrier as claimed in Claim.6, wherein the first phase depth is approximately 1100 whilst the second phase depth is approximately 1801.

8. A record carrier as claimed in Claim 1, in which the two types of information areas have the same dimensions, and the longitudinal direction of the first type of information areas is transverse to that of the second type of information areas.

9. A record carrier as claimed in Claim 1, which is disk-shaped, and in which consecutive track portions within a track revolution differ from each other in that they comprise information areas of the first type and information areas of the second type respectively.

10. A record carrier as claimed in Claim 1, which is disk-shaped, and in which the information structure comprises two spiral tracks, of which the first and the second track respectively comprise information areas of the first and second type, the track revolutions of the first spiral track being interposed between those of the second spiral track.

11. A record carrier as claimed in Claim 1, provided with two information layers, wherein the first information layer comprises only information areas of the first type and the second information layer only information areas of the second type.

12. A record carrier as claimed in Claim 11, wherein the track portions of the first informatiQn layer are situated between those of the second information layer.

13. A record carrier as claimed in Claim 1, provided with two information layers, wherein edch information layer comprises two types of information areas, those track portions of the two information layers which comprise the same type of information areas being interposed between one another.

14. A record carrier as claimed in Claim 8, in which record carrier a user can record useful information in specific record carrier portions, wherein the information already present is servo information in the form of sector addresses included in an optically detectable servo track, which contains addresses of still unrecorded T GB 2 058 434 A 15 1M record carrier portions, which portions contain a material which is inscribable by means of radiation, the longitudinal directions of the information areas in two adjacent sector addresses being transverse to each other.

15. A record carrier as claimed in Claim 8, provided with information recorded by a user, wherein there is provided an optically detectable servo track which includes sector addresses, the information associated with a specific sector address being contained in two information tracks, of which at least one information track is shifted relative to the servo track and transverse to the track direction, and the longitudinal direction of the information areas in one information track being transverse to that of the information areas in the second information track.

16. An apparatus for reading a record carrier as claimed in Claim 1, which apparatus is equipped with an optical read system comprising a radiation source producing a read beam, an objective system for focussing a read beam to a read spot on the information structure, and a radiation-sensitive detection system for converting the read beam which has been 90 modulated by the information structure into an electrical signal, wherein at the location of the information structure the read beam produced by the optical read system comprises two read beam components, which may be present simultaneously or not, whose directions of polarization are transverse to each other and are respectively parallel to or transverse to the longitudinal direction of one type of information areas.

17. An apparatus as claimed in Claim 16, wherein at a location between the radiation source and the objective system there is arranged a half-wave plate which can be moved into and out of the read beam, said half-wave being half 105 the read-beam wavelength.

18. An apparatus as claimed in Claim 16, in which the radiation source is a diode laser arranged so as to be rotatable through an angle of 900.

19. An apparatus as claimed in Claim 16, in which a polarization-sensitive beam splitter is included between the radiation source and the objective system, wherein the direction of polarization of the read beam produced by the source makes an angle of approximately 450 with the longitudinal direction of one type of information area, and wherein between the beam splitter and the objective system there is included a polarization rotator, which alternately rotates the direction of polarization of both the read beam emitted by the radiation source and the read beam reflected by the information structure through an angle of approximately +450 and an angle of approximately -4511.

20. An apparatus as claimed in Claim 16, wherein at the location of the information structure the direction of polarization of the read beam emitted by the radiation source makes an angle of approximately 4511 with the longitudinal130 direction of one type of information area, and wherein the detection system is polarizationsensitive.

21. An apparatus as claimed in Claim 20, wherein the detection system is constituted by a polarization-sensitive beam splitter and two radiation-sensitive detectors disposed in the individual radiation paths of the subbeams formed by the beam splitter.

22. An apparatus as claimed in Claim 16 for writing and reading a record carrier provided with an information layer with an optically detectable servo track in which sector addresses are included, which contain the addresses of associated record carrier portions, which record carrier portions are adapted to convey information, which apparatus comprises a radiation source producing a write beam, an intensity modulator for switching the intensity of the write beam between the first (write) level and a second, lower, level, wherein the write spot which is formed on the information layer by the objective system is elongate and there are provided means for positioning the write spot in two orientations in which the longitudinal directions of the write spot differ by substantially 900 from each other, whilst. said longitudinal directions both make an angle of approximately 451 with the longitudinal direction of the servo track.

23. An apparatus as claimed in Claim 22, wherein said means are constituted by a rotatable element which causes astigmatism and which is arranged in the write beam.

24. An apparatus as claimed in Claim 22, in which the radiation source is a semiconductor diode laser, and said means are constituted by mechanical means for rotating the diode laser about an axis which coincides with the axis of the write beam.

25. An optically readable record carrier substantially as described with reference to Figures 1, 2 and 3a.

26. An optically readable record carrier substantially as described with reference to Figures 1, 2 and 3b.

27. An optically readable record carrier substantially as described with reference to Figures 1, 2 and 4.

28. An optically readable record carrier substantially as described with reference to Figure 5.

29. An optically readable record carrier substantially as described with reference to Figure 6.

30. An optically readable record carrier substantially as described with reference to Figures 7 and 8.

3 1. An optically readable record carrier substantially as described with reference to Figure 9 or Figure 10.

32. An optically readable record carrier substantially as described with reference to Figure 16 or Figure 17.

33. An optical record read apparatus 16 GB 2 058 434 A 16 substantially as described with reference to Figure 10 substantially as described with reference to 11.

18.

34. An optical record read apparatus substantially as described with reference to Figure

35. An optical record read apparatus substantially as described with reference to Figures 18 and 19.

36. An optical record read apparatus Figures 18 and 20.

37. An optical record combined read and write apparatus substantially as described with reference to Figure 21.

38. An optical record combined read and write apparatus substantially as described with reference to Figure 22.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1981. Published by the Patent Office, Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.

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